The aerodynamic drag of high speed trains

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  • Journal of Wind Engineering and Industrial Aerodynamics, 34 (1990) 273-290 Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands

    273

    THE AERODYNAMIC DRAG OF HIGH SPEED TRAINS

    N.J.W. BROCKIE and C.J. BAKER

    Department of Civil Engineering, Nottingham University~ University Park, Nottingham, NG7 2RD (U.K.) (Received December 15, 1988; accepted in revised'form March 6, 1990)

    Summary

    The resistance to forward motion of a high speed passenger train at cruising speed is dominated by aerodynamic drag. To measure train aerodynamic drag a model must be of a small scale in order to fit a standard wind tunnel working section. Due to the scale dependency of the skin friction component of to(~l aerodypAmic drag the results of wind tunnel tests cannot be directly related to train performance at full scale Reynolds number. It is thus necessary to quantify the relat.;enship between skin friction drag and Reynolds number.

    A series of boundary layer and total aerodynamic drag measurements were made using small scale model high speed passenger trains at zero yaw. Boundary layer measurements were also made at full scale to determine skin friction. A simulation program was used to model the recorded boundary layer development and the train boundary layer was found to be strongly three-dlmen- sional. The dependence of train skin friction drag, and hence the total aerodynamic drag, on Reynolds number was clearly demonstrated but the complexity of the train flow regime at full scale prevented an accurate quantification of this.

    The nature of the train boundary layer has been clarified and this knowledge will assist future investigations of high speed passenger train aerodynamic drag.

    Notation

    a, bbb~C A CDO Cv o Cf Co H k L P R Rek

    coefficients in train resistance equation train frontal area total aerodynamic drag coefficient at zero yaw pressure drag coefficient local skin friction coefficient skin friction drag coefficient boundary layer velocity profile shape factor yaw correction factor train length train perimeter train total resistance Reynolds number based on train length

    0167-6105/90/$03.50 1990--Elsevier Science Publishers S.V.

  • 274

    v train velocity relative to the ground V train velocity relative to the ambient wind y coordinate normal to surface Greek letters J* 0 P P zw

    velocity profile displacement thickness velocity profile momentum thickness kinematic viscosity of air density of air wall shear stress train yaw angle

    1. Introduction

    In recent years there has been increasing pressure for improved performance of high speed passenger trains, whether by increasing fuel efficiency or by in- creasing cruising speeds. To assess the performance of present and future train designs requires that the train resistance be known. This is commonly related to the train speed by the equation:

    R=a+ bl v+ b2 V' l 'c(~) V 2 (1)

    Here R is the train total resistance, v is the train velocity relative to the ground, the cvnstants a and bl represent the mechanical resistance effects, the term b2 V corresponds to momentum drag due to infested air, and Vis the train velocity relative to the ambient wind. The term c (~ V 2 is the total aerodyn- amic drag which is a function of the yaw angle, ~P. At train speeds of 250-300 km h- 1 75% of the total resistance to motion is due to aerodynamic drag [ 1 ]. This paper concentrates on high speed passenger train aerodynamic drag.

    The total aerodynamic drag term in eqn. (1) can be written:

    C (~/) -- PCDo ( 1 ~- k ~/) where ~P is measured in radians (2)

    Here A is the train frontal area, CDO is the total aerodynamic drag coefficient at zero yaw, k is the yaw correction factor and p is air density.

    From classical boundary layer theory is known that the skin friction drag, due to the turbulent boundary layer on a fiat plate, decreases with increasing Reynolds number. By the nature of the large length-to-width ratio of a high speed passenger train skin friction drag is a major component of the total aero- dynamic drag. Thus if the train boundary layer flow can be compared with that of a fiat plate turbulent boundary layer, it is to be expected that the train skin friction will display a Reynolds number dependence between model and full scale. It follows that without correction for Reynolds number effects, model scale wind tunnel results cannot be directly applied at full scale because of the larger skin friction contribution to the total aerodynamic drag at model scale. For high speed passenger train aerodynamic drag, investigations are required to enable the relationship between skin friction drag and Reynolds number to be quantified.

  • 275

    The aim of the project reported herein was to establish that relationship by means of model scale tests, full scale tests and computer simulations of mea- sured train boundary layers. Total aerodynamic drag measurements and boundary layer measurements were made on a 1/76th scale HST (British Rail high speed train). Similar measurements were made at 1/40th scale using a different ground effect representation. Boundary layer and skin friction mea- surements were made on a full scale service HST. For greater detail the reader is referred to ref. 2.

    2. Experimental method

    The 1/76th scale tests were performed in the Nottingham University envi- ronmental wind tunnel. An image model technique of ground effect represen- tation was employed with adapted 00-gauge hobbyist models. This technique was chosen, beca-__~ for ~ length trains, if the model is mounted on a f'Lxed ground plane then the ground plane boundary layer growth will submerge the rear part of the train, rendering the model scale skin friction inaccurate. Total aerodynamic drag measurements were made by recording the strain in a ver- tically mounted cantilever as the top train was displaced by the drag load (Fig. 1). Boundary layer measurements were made by hot film anemometry.

    The 1/40th scale tests were performed in the Southampton University low speed wind tunnel. This has a moving ground belt facility which gives good ground effect representation. The t~in underbody flow and skin friction are accurately modelled for the full length of the train. The train total aerodynamic drag was measured by recording the drag of individual vehicles in the train (Fig. 2). Boundary layer measurements were made by flattened pitot tube trav- erses propelled by a stepper motor located within the model.

    At full scale a Mk III laboratory coach was inserted into a service HST. Boundary layer measurements were made by projecting a total pressure rake through the coach side as the train travelled along a virtually straight and level length of track (Fig. 3). The rake w~3 limited to 150 mm in length by the British Rail clearance gauge restrictions. Preston tube measurements were also made to assess the feasibility of this method for the measurement of train skin friction st full scale. Experiments were carried out on the straight, level section of the BR East Coast Main Line between York and Northallerton in Northern

    Flow d,rechon Top trmn

    \ Track- - / _L ..'r ~

    '~Q0e t,.o,, I ! 1

    -- ~ r o d

    _r_ . j . , . ~ . . i /

    , , I -s'rQ,o g og,d 1 t 1 ~ tl co.t,le,er

    t V - - -7"

    Bridge rlrcult balance - ampltflef o~d dtsplny " "

    Fig. I. 1/76th scale wind tupnel .-~. odeL

  • 276

    Foired suspension columns

    ~/ ' [-'-~ Suspension be,~m Susp(

    ~ ~ _ Pressure dueling Strain gauge~r~~ t bmann~ meter

    Loocl measuring DVT ~aessure tapped~BrOU;dory layer

    Load rneosuring coach

    Flow direction ==

    ~enslon wires

    Hoving ground belt

    Fig. 2.1/40th scale wind tunnel model.

    Argus Fig. 3. Boundary layer rake and Preston tubes.

    England. The lab coach was located adjacent to a power car and was the first coach in the rake for southbound runs and the final coach ~vr nc,rthbour~d runs. The ambient wind speed and direction was monitored and virtually still air conditions prevailed during the tests.

    3. Results

    3.1.1/76th scale A large number of identical tests were performed at 1/76th scale in order

    that a repeatable mean value of CDO was obtained. This was

    CDo = 1.85_+ 5%

    The Reynolds number based on train length was Rej = 2.54 106.

  • 277

    Boundary layer mean velocity profiles were measured at sixteen experimen- tal positions on the side and roof of the top train. The mean velocity profile integral parameters are shown in Table 1. The profile positions are identified by the number of the power car (PC) or trailing coach (TC) and a letter F, M or R representing the ~ront, middle or rear respectively of the vehicle side. Figure 4 shows a typical mean velocity profile recorded at position TC4M. At this small scale the first position where a full mean velocity profile could be recorded was at the rear of the leading power car, position PC1R. The devel- opment of the boundary layer along the model was not steady, the profiles recorded at either side of ~he intercoach gaps between coaches I and 2 and

    TABLE 1

    1/76th scale train side boundary layer parameters

    Position Re, J(m) J*(m) 8(m) H Cf

    PC1R 1.67 105 0.008 0.0008 0.0015 1.476 TC1F 2.46 105 0.009 0.0014 0.0010 1.454 TCIM 3.78 105 0.010 0.0016 0.0011 1.442 TC 1R 5.01 105 0.020 0.0023 0.0017 1.392 TC2F 5.41X 105 0.018 0.0023 0.0017 1.381 TC2M 6.75 X 105 0.021 0.0028 0.0020 1.381 TC3M 9.72 X 10 ~ 0.026 0.0026 0.0020 1.304 TC3R 1.10X 10 e 0.030 0.0~29 0.0022 1.305 TC4F 1.14 10 s 0.024 0.0627 0.0021 1.262 TC4M 1.27 X 10 s 0.026 0.0029 0.0023 1.277 TC5M 1.57 10 e 0.038 0.0038 0.0030 1.272 TC6M 1.86 10 e 0.037 0.0042 0.0033 1.295 TC7M 2.16 X

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